[1] Central Research Institute for Physics, Budapest P.O.B. 49, H-1525, Hungary
[2] Institute of Experimental Physics, University of Vienna, Boltzmanngasse 5, A-1090 Vienna, Austria
[3] St. Petersburg State University, Institute of Physics, Department of Optics and Spectroscopy, Ulyanovskaja str. 1, 198904 St. Petersburg, Russia